Subtalar arthroereisis is a surgical operation practiced in the context of curing flexible flatfoot, in particular in children and adolescents.
Flatfoot syndrome is characterised by an instability of the talocalcaneal joint. This instability causes a phenomenon of hyperpronation during the placement of a load on the foot, which leads to a collapse of the plantar arch. The main clinical consequences are:                pain;        walking disorders;        a rigidity of the Achilles tendon during walking;        a deterioration of the peripheral joints due to a modification of the distribution of the loads.        
Subtalar arthroereisis allows the movement of the joint to be limited by causing blocking via an implant.
For not very advanced stages of flatfoot, and in particular when the flatfoot is called “flexible”, arthroereisis is an operation that combines a plurality of clinical benefits with respect to other therapeutic options.
Indeed, arthroereisis is a fast operation that requires a minimal incision and does not lead to the cutting of bone. Arthroereisis also allows a correction obtained to be preserved, even after the extraction of the implant used to carry out the correction. Thus, this is an operation that is reversible in the case of complications via simple removal of the implant as mentioned above.
In order to carry out the arthroereisis, the implant (generally an extra-osseous screw) is inserted into the sinus tarsi oriented in an anterolateral position towards a posterior-medial position. In other words, the implant is inserted into the talocalcaneal joint via its lateral orifice.
After the insertion, the implant must rest on the surface of the calcaneus in a “free float” configuration. After immobilisation, fibrous tissue, called colonising tissue, must colonise rough spots and cavities of the implant in such a way as to keep it in place in situ.
According to a current conventional design, the subtalar implants for arthroereisis of the talocalcaneal joint are in the form of a screw, that is to say, a generally conical oblong shape of revolution provided with a plurality of helical grooves. The helical grooves form a screw pitch that is not intended to move the implant forward by screwing it, but only act as a substrate for the fibrous regrowth of the colonising tissue. Indeed, the colonising tissue attaches in the helical grooves in order to produce a resistance to traction along the axis of insertion of the implant.
This conventional design of the implants has disadvantages, however.
Indeed, recurrent complications leading to the removal of the implant are noted in the literature.
A main complication lies in a backward movement of the implant, and potentially to the creation of a discomfort related to the backwards movement of the implant (inflammation of the sinus tarsi, synovitis, etc.).
The backwards movement of the implant is generally attributed to the shape of the implant. Indeed, during walking, a stress is applied to the generally conical contour of revolution of the implant. This stress takes the form of micromovements and leads to a phenomenon of unscrewing of the implant.
The stresses on the generally conical contour of revolution of the implant are indeed transformed into an axial movement of translation, which leads to the phenomenon of unscrewing because of the helical thread (helical groove or screw pitch).
In order to provide better anchoring for the tissue, a plurality of solutions have been proposed:                the patent document published as U.S. Pat. No. 8,092,547 B2 proposes integrating holes around the periphery of the implant, these holes opening or not opening onto a cannula. The implant thus has a conical shape having an anchoring thread on the radially outer surface of the cone;        the patent document published as US 2009/0099664 A1, proposes a conical implant having, on its radially outer surface, an anchoring thread, and integrating grooves inside the hollows formed by the anchoring thread;        the patent documents published as U.S. Pat. No. 8,092,547 B2 and WO 2012/100054 A1 propose integrating profiles of threads qualified as negative, that is to say, producing an undercut.        
The patent document published as WO 2012/100054 A1, cited above, also proposes an implant that does not have a conical portion and that has, instead of this conical portion, a spheroidal portion so as to provide uniform blocking of the joint during the presence of an anatomical particularity or of the implantation technique of the surgeon. Indeed, this document specifies that a conical implant can easily be implanted in such a way that it produces a pressure that is too low or too high on the talocalcaneal joint.
Finally, the prior art proposes the patent document published as U.S. Pat. No. 9,125,701 B2 that describes an implant manufactured using a plurality of components and that is based on the principle of the screw-expanding anchor. The implementation of such an implant is more complicated and costlier than a machined one-piece implant. Also, mechanical complications caused by the nature of the assembly may occur.
The patent document published as US 2013/0304224 A1 is also known, this document describing an implant comprising a main body and fastening elements independent of the main body. The fastening elements consist of screws to be inserted into the main body, these screws being designed to extend out of the main body in such a way as to be anchored in osseous structures. The implant described in this document is designed to carry out an arthrodesis, that is to say, that its goal is to lead to the creation of osseous fusion in a pathological joint. Such an implant is not suitable for carrying out an arthroereisis.
The goal of the solutions proposed by the prior art is to provide better anchoring for the colonising tissue, but these solutions can nevertheless contribute to the phenomenon of osseous irritation or even erosion. The implants presented by the prior art also constantly introduce a propensity to unscrewing that is not completely solved by the proposed solutions.